Capacitive recording systems have been developed which are capable of storing large volumes of electronic signal information on recorded disc which information can be regenerated by playback of the recorded disc on a capacitive record player. A capacitive playback and recording system utilizing recorded discs is disclosed in U.S. Pat. No. 3,842,194, issued to J. K. Clemens, entitled, "INFORMATION RECORDS AND RECORDING/PLAYBACK THEREFORE".
The stylus which is used in the playback of capacitive recorded disc of the type described above must be electrically conductive. Capacitive styli typically have elongated supporting shafts comprised of a relatively hard wearing material such as sapphire or more preferably diamond which are inherently dielectric in nature. The electrical conductivity required in the pickup stylus can be provided by forming an electrode on a surface of the stylus as disclosed in Clemens in FIG. 5, and as described at column 7, lines 34 to 63. The electrode is formed on the back side of the stylus. The supporting shaft is shaped at one end to a tip.
Defects in the stylus present substantial problems with regard to the playback performance of a recorded disc. The entire capacitive signal information from the playback of the recorded disc is picked up by the stylus. Any defect in the performance of the stylus and particularly in the electrical pickup characteristics of the stylus will cause an aberation of the desired electronic signal over the entire playback of the capacitively recorded disc.
The manufacture of capacitive pickup stylus having a consistent high quality of performance has proven to be a major manufacturing problem. The area in the manufacture of the stylus which has presented the most difficulty is the formation of the electrode on the back side of the stylus. In order to obtain performance of a high quality from a stylus it has been found necessary to form the electrode in a substantial uniform thickness on the stylus.
A method which has been suggested to provide an electrode on the stylus has been to tumble stylus blanks while sputtering them with a metal such as titanium. Such a procedure is disclosed in U.S. Pat. No. 3,325,393, issued to Darrow et al., entitled, "ELECTRICAL DISCHARGE CLEANING AND COATING PROCESS". The tumble sputter method has not proven to be satisfactory as the coating of the metal on the stylus blanks is not uniform. The stylus blanks, when tumbled together even in relatively small numbers, tend to clump together resulting in a random nonuniform application of the sputtered metal on the surfaces of the styli blanks. Certain of the surfaces of the stylus may receive excessively thick amounts of metallic coatings while other surfaces may remain virtually uncoated. Tumble sputtering coating further has a disadvantage that the physical contact of the metallized stylus blank in the tumbling process damages the metal coatings and even causes the styli to adhere to each other. A further problem of tumble sputtering is that all of the surfaces of the stylus blank are coated making it thereafter difficult to select the optimum orientation for micro-machining of the required tips onto the styli. A still further problem with tumble sputtering is that the yield of acceptable coated stylus blanks is inherently low, both due to the relative nonuniformity of the coating on the styli and also because of substantial breakage of the styli during the tumbling.
It would be highly advantageous if a method could be provided for applying electrically conductive coatings to stylus blanks which would be more uniform and in which a higher yield of acceptable styli was obtained. It would be especially advantageous if a method could be provided to selectively and uniformly coat only predetermined surfaces of the styli so as to facilitate further machining of the styli to obtain optimum performance from the finished styli.